There are two common signaling approaches for transmitting data from a transmitter circuit to a receiver circuit. In the first signaling approach, single-ended signaling is used. Single-ended signaling makes use of two conductors between the transmitter circuit and the receiver circuit: a dedicated signal line to send the signal from the transmitter circuit to the receiver circuit, and a common ground return shared by many signals.
In the second signaling approach, differential signaling is used. In differential signaling, true and complement forms of a signal are sent from the transmitter circuit to the transmitter circuit. While this approach also uses two conductors between the transmitter circuit and the receiver circuit, neither conductor is shared with other signals. As such, that differential signaling uses twice as many signal lines as single-ended signaling does.
Differential signaling has a number of advantages over single-ended signaling. One advantage is the ability of the differential receiver to reject signal components that are common to both lines. This ability is referred to as common mode rejection. Since a differential receiver is only sensitive to the difference between its two inputs, and most of the noise that is picked up by the signal lines is common to both lines, the receiver becomes immune to most noise sources. By comparison, with single-ended signaling, the external noise picked up by the signal line cannot be completely separated from the true signal.